Generally, the roll-shaped magnet is fitted in a cylindrical sleeve to cooperate with the latter in transferring toner. There are two types of roll-shaped magnets: a symmetrically magnetized type and an asymmetrically magnetized type.
When the roll-shaped magnet of the symmetrically magnetized type is used, the roll-shaped magnet is rotated in the sleeve so that the toner is transferred along the outer peripheral surface of the sleeve, while, when the roll-shaped magnet is the asymmetrically magnetized type, the sleeve is rotated to transfer the toner.
FIG. 1 shows a typical example of a conventional roll-shaped magnet. This roll-shaped magnet has a shaft 1 and an isotropic ferrite magnet 2 sintered and shaped into a pipe-like form. The isotropic ferrite magnet 2 is bonded to the outer peripheral surface of the shaft 1. The roll-shaped magnet is magnetized through a magnetizing yoke which is adjusted to provide the desired magnetic force for each pole by varying factors such as the shape of the magnetizing yoke, the magnetizing current, the number of turns of the magnetizing winding and so forth.
The roll-shaped magnet produced by the above-stated magnetizing method exhibits a stable magnetic characteristic without a fluctuation of the rate of magnetization, if it is magnetized fully, i.e. to the level of the magnetic saturation. However, the magnetic characteristic is inconveniently rendered unstable when the magnetization is at a level below the magnetic saturation. This poses a problem in the manufacture of the asymmetrically magnetized roll-shaped magnet or roll-shaped magnets having different degrees of magnetization with the same magnetic material, although no problem is created in the manufacture of roll-shaped magnets having a full magnetization of the symmetrical type. In the manufacture of a roll-shaped magnet of the asymmetrically magnetized type, or the roll-shaped magnets having different degrees of magnetization with the same magnetic material, it is often necessary to magnetize the magnetic material to an unsaturated level. In such a case, the degree of magnetization is largely varied by fluctuations of magnetizing factors such as level of the magnetizing current, number of turns of the magnetizing winding and so forth so as to make it difficult to control the desired fluctuation of the magnetic force.
It should also be pointed out that the blank of the roll-shaped magnet exhibits a large deflection due to a contraction caused by the sintering. More specifically, the deflection becomes greater as the axial length of the blank becomes greater and as the diameter of the same becomes smaller. This limits in the practical size of the roll-shaped magnet since a deflection of the magnet blank causes a deformation of the central bore, which in turn deteriorates the tightness of the fit between the shaft 1 and the magnet 2, resulting in a reduced bonding strength. In addition, the magnetic flux density is as small as 1100 to 1150 gauss even at the full magnetization while the magnet has a comparatively large specific gravity of about 1.5 and can easily be broken by an impact.
FIGS. 2 and 3 show another example of conventional roll-shaped magnets in which anisotropic plate-shaped sintered magnets 4 are bonded to the desired pole positions on the surface of the shaft 3. The distances a and b between the center of the shaft 3 and the surfaces of the plate-shaped sintered magnets 4 are suitably adjusted to provide a desired magnetic force distribution.
This type of roll-shaped magnet poses the following problems. The use of anisotropic plate-shaped sintered magnets 4 uneconomically raises the material cost and requires a large number of steps in the manufacturing process. In order to obtain a uniform magnetic force distribution in the axial direction, it is necessary to provide a precise degree of flatness to the surfaces of the plate-shaped magnets, which in turn requires high precision processing such as machining and bonding of the plate-shaped magnets 4. Thus, this type of the roll-shaped magnet is not suitable for mass-production. For producing an axially long roll-shaped magnet of this type, it is necessary to connect a plurality of segments of each plate-shaped sintered magnet in the axial direction to obtain the desired length. As a result, a non-linearity or local reduction of the magnetic force is observed at each seam G of the plate-shaped sintered magnets 4.